1. What other problems will your customer face once they buy the solution from you. It must build on the problem you have just solved and solve the same problem. An example, if you have an issue getting leads. You buy service to get 100 leads from Linkedin. Now, if you try to upsell them 100 leads from Facebook. Guess what. It's not interesting. You are solving the same problem.
If you sell them 'how to convert leads from Linkedin to opportunities' you are solving their next issue. Think about that when approach additional sales from customers.
2. Get them closer to guaranteed success. They have purchased your product or expertise. Hence, you allow them access to that expertise in a product or service outcome. As a next step how can you get them closer to guaranteed success.
Adapt those thoughts into your sales process and see how you will drive conversions as a result.